Preloading Instructions from an Instruction Set Other than a Currently Executing Instruction Set

ABSTRACT

A preload instruction in a first instruction set is executed at a processor. The preload instruction causes the processor to preload one or more instructions into an instruction cache. The pre-loaded instructions are pre-decoded according to a second instruction set that is different from the first instruction set. The preloaded instructions are pre-decoded according to the second instruction set in response to an instruction set preload indicator (ISPI).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation patent application of, and claimspriority from, U.S. patent application Ser. No. 11/685,850, filed onMar. 14, 2007, and entitled “SYSTEM, METHOD AND SOFTWARE TO PRELOADINSTRUCTIONS FROM AN INSTRUCTION SET OTHER THAN ONE CURRENTLYEXECUTING,” which is hereby incorporated by reference in its entirety.

FIELD

The present invention relates generally to the field of processors and,in particular, to pre-loading and pre-decoding instructions according toan instruction set other than a currently executing instruction set.

BACKGROUND

Microprocessors perform computational tasks in a wide variety ofapplications. Improved processor performance is almost always desirable,to allow for faster operation and/or increased functionality throughsoftware enhancements. By exploiting both architectural advances, suchas RISC architectures, and advances in semiconductor technology, manymodern processors execute at clock speeds much higher than memory chips,such as DRAM and SDRAM. To minimize the penalty of relatively slowmemory accesses, these processors utilize a hierarchical memorystructure, with fast, on-chip cache memories storing local copies ofdata and instructions that have been recently accessed and/or that theprocessor anticipates (via software) accessing in the near future.

One example of anticipating loads to an instruction cache is a preloadinstruction (e.g., the ARM instruction PLI). A preload instruction loadsinstructions into the instruction cache in advance of their execution. Aprogrammer may insert a preload instruction into code somewhat ahead ofa known or anticipated branch, such as a subroutine call, to a target.The preload instruction loads instructions at the target into theinstruction cache, so that when the branch executes, instructions at thetarget are available in the cache for immediate execution. Instructionsto be loaded into a cache in response to a preload instruction arereferred to herein as pre-loaded instructions.

Most modern processors employ a pipelined architecture, where sequentialinstructions are overlapped in execution to increase overall processorthroughput. Maintaining smooth execution through the pipeline iscritical to achieving high performance. One pipeline optimizationtechnique known in the art is pre-decoding instructions. Instructionsare examined as they are read from memory, are partially decoded, andsome information about the instructions—known as pre-decodeinformation—is stored in an instruction cache along with the associatedinstructions. When the instructions are later fetched into the pipelinefrom the cache, the pre-decode information is also fetched, and is usedto assist in fully decoding the instructions. Pre-decode informationmay, for example, identify branch and floating-point instructions,identify the length of an instruction in a variable-length instructionset architecture, or other information useful in executing theinstructions in the pipeline.

Some modern processors are able to execute instructions according to twoor more instruction set encodings. For example, a processor may have a32-bit native instruction set, and additionally include the ability toexecute Java byte codes. As another example, several versions of the ARMprocessor architecture execute 32-bit ARM instruction encodings in anARM instruction set operating mode, and 16-bit or 32-bit Thumbinstruction encodings in a Thumb instruction set operating mode.Programmers explicitly set the proper instruction set operating mode insoftware, prior to executing instructions according to the relevantinstruction set encoding.

When a conventional processor having a pre-decoder executes a preloadinstruction, the pre-loaded instructions are pre-decoded according tothe current instruction set operating mode. If a programmer wishes topreload instructions in an instruction set different from the currentprocessor instruction set operating mode—for example in anticipation ofa branch to code section written in a different instruction set—theprogrammer must change the instruction set mode prior to executing thepreload instruction.

For example, a program executing ARM instructions in ARM mode may changeto Thumb mode and branch to a code segment executing only Thumbinstructions. To preload the Thumb instructions with correctpre-decoding, the program must switch the current processor instructionset operating mode from ARM to Thumb, execute a preload instruction withthe target address of the Thumb code, then switch back to ARM mode toexecute the remaining ARM instructions, prior to the branch into theThumb code section.

Failure to enter Thumb mode to execute the preload instruction wouldcause the pre-decoder to incorrectly pre-decode the pre-loaded Thumbinstructions according to ARM instruction set encodings. If thisoccurred, when fetched from the instruction cache the pre-decodeinformation would be useless and the Thumb instructions would have to bere-fetched, incurring a delay that the preload was meant to obviate. Inaddition, in an instruction cache having an ARM/Thumb bit as part of thetag, the re-fetched Thumb instructions would be allocated a separatecache line, and the Thumb instructions incorrectly pre-decoded as ARMinstructions would remain in the cache, wasting cache space and hencelowering the cache hit rate and system performance.

In many processors, the overhead involved in switching the currentprocessor instruction set operating mode twice to execute a singlepreload instruction may outweigh the benefit of pre-loading instructionsin the anticipated instruction set.

SUMMARY

According to one or more embodiments disclosed herein, a preloadinstruction in a first instruction set is executed at a processor. Thepreload instruction causes the processor to preload one or moreinstructions into an instruction cache. The pre-loaded instructions arepre-decoded according to a second instruction set that is different fromthe first instruction set. The preloaded instructions are pre-decodedaccording to the second instruction set in response to an instructionset preload indicator (ISPI).

One embodiment relates to a method of preloading instructions. Themethod includes, at an instruction execution unit of a processorexecuting in a first instruction set operating mode associated with afirst instruction set of a plurality of instruction sets, generating aninstruction set preload indicator identifying a second instruction setof the plurality of instruction sets. The second instruction set is asubset of the first instruction set. The method also includes executing,in accordance with the first instruction set operating mode, a preloadinstruction operative to preload an instruction into an instructioncache. The method further includes pre-decoding the pre-loadedinstruction in the instruction cache according to a second instructionset operating mode associated with the identified second instructionset.

Another embodiment relates to a processor. The processor includes amemory interface and an instruction cache operative to storeinstructions and pre-decode information. The processor also includes aninstruction execution unit operative to retrieve the instructions whenthe processor is in a first instruction set operating mode and togenerate an instruction set preload indicator. The processor furtherincludes a pre-decoder interposed between the instruction cache and thememory interface. The pre-decoder is operative to pre-decode pre-loadedinstructions according to a second instruction set operating mode inresponse to the instruction set preload indicator. The first instructionset operating mode is associated with a first instruction set of theprocessor and the second instruction set operating mode is associatedwith a second instruction set of the processor. The first instructionset has a first code density that is different than a second codedensity of the second instruction set.

Another embodiment relates to a computer-readable medium. Thecomputer-readable medium stores a preload instruction in a firstinstruction set. The preload instruction is executable by a processor tocause the processor to preload instructions into an instruction cache.The pre-loaded instructions are pre-decoded according to a secondinstruction set that is a reduced-size version of the first instructionset.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a particular embodiment of a processorcapable of pre-decoding pre-loaded instructions according an instructionset other than a current processor instruction set mode; and

FIG. 2 is a flow diagram of a particular embodiment of a method ofpre-loading instructions in one of a plurality of instruction sets.

DETAILED DESCRIPTION

FIG. 1 depicts a block diagram of a processor 110 that executesinstructions encoded according to a plurality of different instructionset encodings. The processor 110 is capable of pre-decoding pre-loadedinstructions according an instruction set other than the currentprocessor instruction set mode.

The processor 110 executes instructions in an instruction execution unit112 that includes a pipeline 114 comprising a plurality of registers orlatches 116, organized in pipe stages, as well as logical andcomputational circuits such as Arithmetic Logic Units (ALU) (not shown).The pipeline executes instructions according to control logic 118. Thepipeline 114 may be a superscalar design, as shown. A General PurposeRegister (GPR) file 120 provides registers comprising the top of amemory hierarchy. The instruction execution unit also includes a statusregister 122 that indicates, for example, the current processorinstruction set operating mode, which defines one of a plurality ofinstruction set encodings under which instructions are decoded andexecuted. The instruction execution unit 112 also includes L2 processinglogic 124 that tracks properties of instructions and data fetched froman L2 cache memory 136 or main memory 138.

The pipeline 114 fetches instructions from an instruction cache 126,with memory addressing and permissions managed by an Instruction-sideTranslation Lookaside Buffer (ITLB) 128. A pre-decoder 130 pre-decodesinstructions fetched from the L2 cache 136 (or main memory 138) andplaces the instructions and pre-decode information into the instructioncache 126. As discussed in greater detail herein, the pre-decoder 130pre-decodes pre-loaded instructions according to an instruction setdifferent than the current processor instruction set operating mode, inresponse to an instruction set preload indicator (ISPI) from theinstruction execution unit 112.

Data is accessed from a data cache 132, with memory addressing andpermissions managed by a main Translation Lookaside Buffer (TLB) 134. Invarious embodiments, the ITLB 128 may comprise a copy of part of the TLB134. Alternatively, the ITLB 128 and TLB 134 may be integrated.Similarly, in various embodiments of the processor 110, the instructioncache 126 and data cache 132 may be unified.

Misses in the instruction cache 126 and data cache 132 cause an accessto Level-2 (L2) cache memory 136 which, in the embodiment depicted inFIG. 1, is integrated. Misses in the L2 cache 136 cause an access tomain (off-chip) memory 138 over a system bus 140, under the control of amemory interface 142. The processor 110 may include an Input/Output(I/O) interface 144, controlling access to various peripheral devices146, 148 across a peripheral bus 150. As well known in the art, theprocessor 110 executes instructions retrieved from a computer readablemedium, such as memory 136, or magnetic or optical media read by aperipheral 146, 148.

Those of skill in the art will recognize that numerous variations of theprocessor 110 are possible. For example, the processor 110 may include aLevel-3 (L3) or additional cache, and/or a separate L2 cache 136 may bededicated to each of the instruction and data caches 126, 132. Inaddition, one or more of the functional blocks depicted in the processor110 may be omitted from a particular embodiment.

The processor 110 executes instructions according to the currentprocessor instruction set mode, as indicated for example in the statusregister 122. To allow the pre-loading—with correct pre-decoding—ofinstructions in a different instruction set, the instruction executionunit 112 generates an instruction set preload indicator (ISPI), andprovides the ISPI to the pre-decoder 130 upon executing a preloadinstruction. If the processor 110 supports two instruction set encodings(e.g., ARM and Thumb), the ISPI may comprise a single bit. If theprocessor 110 supports more than two instruction set encodings, the ISPImay comprise a multi-bit value, and may comprise a separate flag foreach instruction set, or may encode an instruction set indication in abinary value, as well known in the art. The ISPI may be generated in avariety of ways.

In one embodiment, one or more unused address bits of the preloadinstruction target address are encoded as the ISPI. A preloadinstruction (e.g., PLI) takes a target address as an operand. Theinstruction address may be decomposed into, for example, a tag, a setnumber, and a cache line offset. For example, for a 32-bit instructionvirtual address VA[31:0], the tag, which is compared against tags storedin a CAM structure of the instruction cache 126, may comprise VA[31:11].In an n-way set associative cache, the set number selects from among nresults returned if the tag matches. The set number may compriseVA[10:5]. The offset, VA[4:0], addresses individual instructions in thecache line, that are stored in a RAM structure of the instruction cache126 (along with pre-decode information).

A preload instruction loads a fixed-sized block of memory into eachcache line. If all of the instructions are of the same length, a fixednumber of instructions are loaded into the cache line. In avariable-length instruction set, or if the cache line includesinstructions encoded according to different instructions sets havingdifferent length instructions, the number of instructions in the cacheline is not known upon loading the instructions into the instructioncache. In either event, however, the amount of data to be loaded intothe instruction cache line is known, and accordingly the preloadinstruction does not utilize the cache line offset portion of theaddress.

According to one embodiment, the cache line offset, e.g., VA[4:0], isused to encode the instruction set preload indicator (ISPI). Inparticular, in an embodiment where the processor 110 executesinstructions from two instruction sets, the ISPI is the leastsignificant bit VA[0]. As one non-limiting example, VA[0]=0 may indicateARM instructions, and VA[0]=1 may indicate Thumb instructions. Ofcourse, other ISPI encodings are possible, and depend on the number andtype of instruction sets the processor 110 supports.

The ISPI is encoded into the cache line offset portion of a preloadtarget address when the address is determined and written to acompiler-specified GPR in preparation for execution of an associatedpreload instruction. When the preload instruction executes, it reads atarget address from the specified GPR register, and preloads a block ofdata at the target address into a cache line determined by the tag andset number portions of the address. If the address hits in theinstruction cache, the preload instruction execution is complete. If theaddress misses in the instruction cache, logic within the instructionexecution unit 112, such as L2 processing logic 124, performs missprocessing, accessing the L2 cache 136 and, if necessary, main memory138 to retrieve the instructions.

When a block of data containing instructions is loaded into thepre-decoder 130, the L2 processing logic 124 presents the ISPI to thepre-decoder 130. In response to the ISPI, the pre-decoder 130pre-decodes instructions according to the indicated instruction set,regardless of the current instruction set operating mode of theprocessor 110. The pre-decoder 130 then writes the instructions, alongwith the correct pre-decode information, to the instruction cache 126.In one embodiment, the pre-decoder 130 receives an indication of apreload instruction execution and examines the ISPI in response to suchindication (ignoring the ISPI for normal instruction cache linefetches). In another embodiment, logic in the instruction execution unit112, such as the L2 processing logic 124, may combine the ISPI withknowledge of a preload instruction execution, and present to thepre-decoder 130 an indication of the instruction set by which currentlyfetched instructions should be pre-decoded. In either case, thepre-decoder 130 pre-decodes pre-loaded instructions according to aparticular instruction set in response to the ISPI.

In one embodiment, the above-described pre-decoding of pre-loadedinstructions according to the ISPI is selectively enabled, to providebackward compatibility. In particular, an interworking mode is defined.The interworking mode may be indicated by a bit in the status register122. When the interworking mode is enabled, the pre-decoding ofpre-loaded instructions depends on the ISPI, as described above. Whenthe interworking mode is disabled, instructions are pre-decodedaccording to the processor instruction set mode that is active when thepreload instruction is executed.

In one embodiment, the instruction set preload indicator (ISPI)comprises one or more bits in a register, such as the status register122. In this embodiment, the ISPI bits are explicitly set prior toexecution of a preload instruction. Upon execution of the preloadinstruction, the pre-decoder 130 inspects the ISPI bits in the statusregister 122 or other register, and pre-decodes the pre-loadedinstructions according to the instruction set encoding indicated by theISPI bits (regardless of the processor instruction set mode that isactive at the time). The instructions and pre-decode information arethen written to the instruction cache 126. In one embodiment, thepre-decoding of pre-loaded instructions according to the ISPI isselectively enabled by an interworking mode, as described above. In oneembodiment, the ISPI bits may be gated by the interworking modeindicator bit.

In one embodiment, the instruction set preload indicator (ISPI)comprises one or more bits in the op code of the preload instruction. Abit field of the preload instruction op code may be defined for theISPI, and a compiler may set the ISPI bit field according to aprogrammer's instructions or directives in code. Upon execution of thepreload instruction, the ISPI bit field is presented to the pre-decoder130, such as by the L2 processing logic 124, and the pre-decoder 130pre-decodes the pre-loaded instructions according to the indicatedinstruction set encoding. The instructions and pre-decode informationare then written to the instruction cache 126.

In one embodiment, two or more separate preload instructions are definedfor preloading instructions in different instructions sets. As anon-limiting example, an ARM instruction set may include a conventionalpreload instruction that preloads data into the instruction cache 126,with the pre-decoder 130 pre-decoding the pre-loaded instructions as ARMinstructions. In addition, the ARM instruction set may also include aseparate, alternate instruction set preload instruction (e.g., PLI2)that preloads data into the instruction cache 126, with the pre-decoder130 pre-decoding the pre-loaded instructions as Thumb instructions.Similarly, the Thumb instruction set may include separate preloadinstructions for pre-loading data that is pre-decoded according to Thumbor ARM instruction set encodings. This allows programmers to explicitlypreload instructions from the desired instruction set by utilizing theappropriate preload instructions in code.

Those of skill in the art will recognize that the above-describedembodiments are not mutually exclusive. For example, a preloadinstruction having an ISPI bit field in the op code or a unique preloadinstruction may, in particular implementations, present the ISPI to thepre-decoder 130 by latching the ISPI into a register such as the statusregister 122, or by substituting the ISPI into one or more unusedaddress bits in the preload target address. In general, given theteachings of the present disclosure, those of skill in the art mayascertain the best solution for preloading instructions in differentinstruction sets for a given application.

FIG. 2 depicts a method 200 of preloading instructions. The methodbegins by setting an instruction set preload indicator (ISPI) (block202). The ISPI uniquely identifies one of a plurality of instructionsets supported by the processor 110. In various embodiments, the ISPImay be set by altering a preload instruction target address, settingISPI bits in a register such as the status register 122, setting an ISPIbit field in a preload instruction op code, or selecting a uniquepreload instruction.

A preload instruction is executed in the current processor instructionset mode (block 204). The preload instruction is operative to loadinstructions into the instruction cache 126 in advance of the pre-loadedinstructions' execution. In one embodiment, pre-decoding of thepre-loaded instructions depends on the interworking mode. If theinterworking mode is not enabled (block 206), the pre-decoder 130pre-decodes the pre-loaded instructions according to the currentinstruction set mode (block 208). If the interworking mode is enabled(block 206), and the ISPI indicates or identifies an instruction setother than the current instruction set mode (block 210), the pre-decoder130 pre-decodes the pre-loaded instructions according to the instructionset indicated by the ISPI (block 212).

According to one or more embodiments described and claimed herein,programmers may preload instructions into an instruction cache 126 whileexecuting instructions from a first instruction set, with the pre-loadedinstructions pre-decoded according to a second, different instructionset. In this manner, the execution of code that switches betweeninstruction sets may be optimized by ensuring that instructions in agiven instruction set are available in an instruction cache 126 prior tobranching to code written in that instruction set. The communication toa pre-decoder 130 of an instruction set preload indicator (ISPI) may beimplemented in a variety of ways.

Although the present invention has been described herein with respect toparticular features, aspects and embodiments thereof, it will beapparent that numerous variations, modifications, and other embodimentsare possible within the broad scope of the present invention, andaccordingly, all variations, modifications and embodiments are to beregarded as being within the scope of the invention. The presentembodiments are therefore to be construed in all aspects as illustrativeand not restrictive and all changes coming within the meaning andequivalency range of the appended claims are intended to be embracedtherein.

1. A method of preloading instructions, the method comprising: at aninstruction execution unit of a processor executing in a firstinstruction set operating mode associated with a first instruction setof a plurality of instruction sets, generating an instruction setpreload indicator identifying a second instruction set of the pluralityof instruction sets, wherein the first instruction set includes at leastone instruction that is not included in the second instruction set;executing, in accordance with the first instruction set operating mode,a preload instruction operative to preload an instruction into aninstruction cache; and pre-decoding the pre-loaded instruction in theinstruction cache according to a second instruction set operating modeassociated with the identified second instruction set.
 2. The method ofclaim 1, wherein the pre-loaded instruction is a single instruction thatis pre-decoded at a pre-decoder of the processor, the method furthercomprising receiving the generated instruction set preload indicator atthe pre-decoder of the processor.
 3. The method of claim 1, wherein theinstruction set preload indicator comprises one or more bits of anaddress of a cache line into which instructions are preloaded.
 4. Themethod of claim 3, wherein the instruction set preload indicatorcomprises a least significant bit of the address of the cache line. 5.The method of claim 1, wherein the instruction set preload indicatorcomprises one or more bits in an op code of the preload instruction. 6.The method of claim 1, wherein the preload instruction is a uniqueinstruction in the first instruction set that causes pre-loadedinstructions to always be pre-decoded according to the secondinstruction set, and wherein generating the instruction set preloadindicator comprises selecting the unique preload instruction.
 7. Themethod of claim 1, wherein generating the instruction set preloadindicator comprises setting one or more bits in a register prior toexecuting the preload instruction.
 8. The method of claim 1 wherein thepre-loaded instruction is pre-decoded in according to the secondinstruction set operating mode in an interworking mode.
 9. The method ofclaim 1, wherein the first instruction set is an ARM-type instructionset and the second instruction set is a Thumb-type instruction set, orwherein the first instruction set is a Thumb-type instruction set andthe second instruction set is an ARM-type instruction set.
 10. Themethod of claim 9, wherein the at least one instruction not included inthe Thumb-type instruction set is a swap (SWP) instruction, and whereinthe at least one instruction not included in the ARM-type instructionset is an if-then (IT) instruction.
 11. The method of claim 1, whereinthe first instruction set is associated with a first instruction lengthand wherein the second instruction set is associated with a secondinstruction length that is different than the first instruction length.12. The method of claim 10, wherein the first instruction length is a32-bit length and wherein the second instruction length is a 16-bitlength.
 13. The method of claim 1, wherein each instruction in the firstinstruction set has a first instruction length, and wherein the secondinstruction set includes a first instruction having the firstinstruction length and a second instruction having a second instructionlength that is different than the first instruction length.
 14. Themethod of claim 1, wherein the first instruction set comprisesfixed-size instructions and wherein the second instruction-set comprisesvariable-sized instructions.
 15. A processor, comprising: a memoryinterface; an instruction cache operative to store instructions andpre-decode information; an instruction execution unit operative toretrieve the instructions when the processor is in a first instructionset operating mode and to generate an instruction set preload indicator;and a pre-decoder, interposed between the instruction cache and thememory interface, operative to pre-decode pre-loaded instructionsaccording to a second instruction set operating mode in response to theinstruction set preload indicator, wherein the first instruction setoperating mode is associated with a first instruction set of theprocessor, wherein the second instruction set operating mode isassociated with a second instruction set of the processor, and whereinthe first instruction set has a first code density that is differentthan a second code density of the second instruction set.
 16. Theprocessor of claim 15, wherein the instruction execution unit generatesthe instruction set preload indicator by setting one or more bits of anaddress of a cache line of the instruction cache into which instructionsare preloaded.
 17. The processor of claim 16, wherein the address of thecache line is written to a general purpose register.
 18. The processorof claim 15, further comprising a register, wherein the instructionexecution unit generates the instruction set preload indicator bysetting one or more bits in the register.
 19. The processor of claim 15,further comprising a second cache memory interposed between theinstruction cache and the memory interface, wherein the pre-decoder isinterposed between the instruction cache and the second cache memory.20. The processor of claim 15, wherein the pre-decoder is operative topre-decode instructions according to the second instruction setoperating mode in an interworking mode.
 21. A computer-readable mediumstoring a preload instruction in a first instruction set, wherein thepreload instruction is executable by a processor to cause the processorto preload instructions into an instruction cache, wherein thepre-loaded instructions are pre-decoded according to a secondinstruction set, and wherein the second instruction set includes atleast one instruction that is not included in the first instruction set.22. The computer-readable medium of claim 21, wherein each of thepre-loaded instructions is pre-decoded according to the secondinstruction set.
 23. The computer-readable medium of claim 21, whereinthe instruction set preload indicator is stored in a status registerprior to execution of the preload instruction.
 24. The computer-readablemedium of claim 21, wherein the first instruction set is an ARMinstruction set and wherein the second instruction set is a Thumbinstruction set, or wherein the first instruction set is a Thumbinstruction set and wherein the second instruction set is an ARMinstruction set.